CPU Time and Scheduling

Scheduler Execution Frequency

  • The frequency of running the scheduler impacts CPU time utilization.

    • Concept: More frequent scheduler executions waste CPU time on running the scheduler itself, instead of application processes.

    • Alternative Question: How long should a process run?

    • Implication: Longer process run times result in less frequent scheduler invocations.

Understanding CPU Utilization

  • To assess CPU utilization, perform the following calculations:

    • Total Processing Time: During a defined interval, processing time = $2 imes tp$ (where $tp$ is the time allocated to a process that has been scheduled).

    • Total Duration of the Interval: Total duration = $2 imes tp + 2 imes t{sketch}$ (where $t_{sketch}$ is the time taken by the scheduler).

    • Utilization Calculation:

    • CPU Utilization formula:
      extCPUUtilization=extTotalProcessingTimeextTotalDuration=2×t<em>p2×t</em>p+2×tsketchext{CPU Utilization} = \frac{ ext{Total Processing Time}}{ ext{Total Duration}} = \frac{2 \times t<em>p}{2 \times t</em>p + 2 \times t_{sketch}}

    • Example Impact: If processing time equals scheduling time (i.e., $tp = t{sketch}$), this implies:

    • CPU Utilization = 50%

      • This scenario indicates that half of the CPU time is dedicated to system processing work (overhead), not application processing.

Interval Performance Analysis

  • Second Interval Analysis: Consider scenarios where $tp$ is significantly larger than $t{sketch}$ (e.g., $tp = 10 imes t{sketch}$)

    • Utilization Improvement:

    • The CPU utilization in this case is approximately 91%.

    • Efficiency Indicator: This indicates significantly better CPU efficiency as most of the time is spent performing useful work.

Time Slices and Scheduling Design Decisions

  • Time Slice Definition:

    • Time slice ($t_p$) refers to the allocated processing time for a scheduled process to consume on the CPU.

  • Design Trade-offs: Several critical decisions must be made in scheduler design:

    • Appropriate values for time slices.

    • Useful metrics for determining which process to run next.

  • Further Discussion: Design issues related to the scheduler will be covered in subsequent lessons, highlighting the complexity of scheduler implementation.

Summary

  • In summary, the management of how frequently the scheduler runs versus how long each process runs affects CPU utilization. The goal is to optimize the scheduling to enhance the efficiency of CPU usage in executing useful application processes instead of overhead system processes.